A network domain is a collection of network elements within a common sphere of address management or routing procedure which are operated by a single organization or administrative authority. Examples of such domains include interior gateway protocol (IGP) areas, such as Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (ISIS) areas, and Autonomous Systems. A network running a Border Gateway Protocol (BGP) is organized as multiple Autonomous Systems, each of which has a number of IGP areas. The concepts of Software Defined Networks (SDN) reduce the overall network Capital Expenditure (CapEx) and Operational Expenditure (OpEx), whilst facilitating the deployment of services and enabling new features. The principles of SDN include centralized control to allow optimized usage of network resources and provisioning of network elements across domains.
For a network with a number of domains, multiple SDN controllers may be employed with each of the domains in the network controlled by an SDN controller. Architectures of controllers for achieving a centralized control on the network include a hierarchical architecture, a distributed architecture, and a hybrid of a hierarchical and a distributed architecture of controllers. At a top level of a hierarchical architecture is a parent controller that is not a child controller. The parent controller controls a number of child controllers, some of which are not parent controllers, and each of which controls a domain. At lower levels of the hierarchical architecture, some child controllers are both parent controllers and child controllers, each of which controls multiple child controllers, and so on.